Vehicle braking system

ABSTRACT

A dual purpose hydraulic accumulator is adapted to store hydraulic fluid and to release a spring to apply vehicle brakes mechanically in the event of loss of hydraulic pressure. A cylindrical cavity in a fixed body member is divided by a free piston into a pressure chamber for hydraulic fluid and a spring chamber, preferably a gas cushion. A second piston exposed to one of these chambers serves to compress the brake spring. The dual purpose accumulator may be fitted to a trailer having hydraulic brakes and adapted to be hitched to a tractor. A metering valve on the tractor serves to controllably connect the accumulator to the hydraulic actuators. The tractor and trailer lines are releasably interconnected by a quick-release, twinline, self-sealing hydraulic coupling. The accumulator and brake lines on the trailer are automatically interconnected to apply the trailer brakes hydraulically in the event of trailer breakaway. The accumulator can be charged from a constant volume hydraulic pump utilizing an unloader valve.

United States Patent England et al.

154] VEHICLE BRAKING SYSTEM [72] Inventors: Michael John England, 15Eastleigh Croft, Walmley Sutton Coldfield; Derrick Donnell, 73 RosslynAve., Coundon Conventry; Eric Charles Hales, l6 Gunners Lane, Studley,all of England [22] Filed: Oct. 8, 1970 [21] Appl. No.: 79,245

Related US. Application Data [62] Division of Ser. No. 795,104, Jan. 29,1969,

abandoned.

[52] US. Cl. ..303/7, 92/134, 138/31, 188/3, 188/106 P, 188/170, 303/9,303/10 [51] Int. Cl. ..B60t 13/14 [58] Field of Search ..138/31;92/130-134; 303/2, 7, 9-10; 188/3, 106 P, 170

[56] References Cited UNITED STATES PATENTS 3,110,522 11/1963 Bunch..303/9 3,114,580 12/1963 MacDuff ..303/9 X 3,463,276 8/1969 Brooks..303/9 X 3,543,512 12/1970 England et al. ..303/7 X r i E l l l l [451Oct. 3, 1972 Primary Examiner-Milton Buchler Assistant Examiner-John J.McLaughlin Attorney-Holcombe, Wetherill & Brisebois ABSTRACT A dualpurpose hydraulic accumulator is adapted to store hydraulic fluid and torelease a spring to apply vehicle brakes mechanically in the event ofloss of hydraulic pressure. A cylindrical cavity in a fixed body memberis divided by a free piston into a pressure chamber for hydraulic fluidand a spring chamber, preferably a gas cushion. A second piston exposedto one of these chambers serves to compress the brake spring.

The dual purpose accumulator may be fitted to a trailer having hydraulicbrakes and adapted to be hitched to a tractor. A metering valve on thetractor serves to controllably connect the accumulator to the hydraulicactuators. The tractor and trailer lines are releasably interconnectedby a quick-release, twinline, self-sealing hydraulic coupling. Theaccumulator and brake lines on the trailer are automaticallyinterconnected to apply the trailer brakes hydraulically in the event oftrailer breakaway. The accumulator can be charged from a constant volumehydraulic pump utilizing an unloader valve.

12 Claims, 4 Drawing Figures VEHICLE BRAKING SYSTEM RELATED APPLICATIONThis application is a division of our application, Ser. No. 795104,filed Jan. 29, 1969, now abandoned.

The invention relates to vehicle braking systems.

According to one aspect of the invention, a hydraulic accumulatorcomprises a stressed spring acting mechanically to apply the brakes anda piston and cylinder mechanism adapted by the pressure of pressuremedium contained in an accumulator chamber in the cylinder to furtherstress the spring for relieving the mechanically applied brake force,the pressure in the accumulator chamber acting upon further resilientmeans, such as a cushion of gas, to provide hydraulic energy for normaloperation of the brakes.

According to another aspect of the invention, a dual purpose hydraulicaccumulator comprises a fixed body member having a cavity thereindivided by a movable partition member into a pressure chamber for thehydraulic fluid to be stored and a spring chamber acting on said movablepartition member to hold the hydraulic pressure, a brake spring formechanically applying brakes of the vehicle and a brake-release pistonslidable in said body member and exposed to one of said chambers, saidbrake-release piston acting on said brake spring to compress the brakespring and so relieve the brakes of mechanical actuation when thepressure chamber is charged with hydraulic fluid.

Preferably the movable partition member comprises a free piston slidablein said cavity.

According to another aspect of the present invention, a vehicle brakingsystem comprises a hydraulic accumulator adapted to mechanically applybrakes of the vehicle when the accumulator pressure falls below apredetermined value and at least one hydraulic actuator for vehiclebrakes (preferably the previously mentioned brakes), the accumulator andthe actuator having separate lines leading to a metering valve adaptedto connect said lines to one another responsively to an applied signalwhereby pressure medium from the accumulator is used to operate theactuator.

According to a further aspect of the present invention, a trailervehicle braking system comprises a pressure accumulator adapted tomechanically apply brakes of the trailer vehicle when the accumulatorpressure falls below a predetermined value and at least one pressuremedium actuator for brakes of the trailer vehicle, the accumulator andthe actuator having separate lines leading to respective coupling headson the trailer vehicle which are adapted to be coupled to correspondingheads on a tractor vehicle fitted with a metering valve adapted toconnect the tractor vehicle coupling heads to one another responsivelyto an applied signal in order to use pressure medium from theaccumulator to operate the actuator.

Preferably said trailer vehicle coupling heads are fitted withself-closing valves which automatically close when the trailer vehiclecoupling heads become detached from the tractor vehicle coupling headsand a valve arrangement is provided on the trailer vehicle, preferablyadjacent the coupling heads, for interconnecting said lines on thetrailer vehicle in the event that the coupling heads become detachedfrom one another inadvertently.

The present invention is applicable to a vehicle fitted with a constantvolume hydraulic pump for operating hydraulic services through opencentre or open neutral valves and provided with an unloader valveconnected in series with the valves of the hydraulic services andserving to charge the hydraulic accumulator, the unloader valve beingadapted to switch to a pump unloading position after the hydraulicaccumulator has been charged to a predetermined pressure by the pump.

The invention is further described, by way of example, with reference tothe accompanying drawings in which:

FIG. 1 illustrates that part of a vehicle braking system constructed inaccordance with the invention mounted on a tractor,

FIG. 2 illustrates that part of the braking system of FIG. I mounted ona trailer to be coupled to the tractor, and

FIG. 3 and 4 are views similar to FIGS. 1 and 2, respectively, of asecond embodiment of vehicle braking system.

Referring to FIGS. 1 and 2 of the drawings, a vehicle braking system fora trailer receives its supply of hydraulic fluid from a hydraulic pump10 fitted to a tractor to which the trailer is intended to be hitched.

The pump 10 serves to provide basic tractor hydraulic services withhydraulic fluid and operates with constant delivery. A single actinghydraulic cylinder 11 is shown in FIG. 1 by way of example fed from atwo-position control valve 12 which has an open-neutral position, i.e.,in its neutral position it discharges the fluid fed via a line 13 to itsinlet port by the pump 10 to a line 14. In practice a number ofhydraulic services will be provided on the tractor and in most casesthese will comprise double acting devices controlled by threepositionopen-center valves. The valves are connected in series in such a waythat when one valve in the series is in its open-center or open-neutralposition it passes the fluid fed from the pump 10 to the next valve inthe series. In other words the line 14 instead of leading away from thehydraulic services as illustrated would be connected to the inlet portof the next valve in the series. Thus when all the valves are in theiropen-center or open-netural positions the pump 10 discharges directly tothe line 14 which conducts the discharged fluid for return to areservoir 15, from which the pump draws hydraulic fluid through a line16. When one of the valves, such as the valve 12, is operated, the fluiddischarging through the line 14 is throttled to develop the pressure inthe line 13 and the fluid delivered by the pump 10 is delivered at thiscontrolled pressure to the hydraulic cylinder 11. The cylinder 11 inconnected by the valve 12 to a return line 17 leading to the reservoir15 when the valve 12 is in its neutral position illustrated. Thus thehydraulic system for the tractor services is basically a constant volumesystem in which the pressure varies according to the demand by theservices. However to operate the trailer brakes a source of hydraulicfluid under constant pressure is required and this source is obtainedfrom the pump 10 by means of an unloader valve 18 whose inlet port 19 isconnected to the line 14.

The trailer braking system shown in FIG. 2 comprises a hydraulicaccumulator and at least one hydraulic actuator, two such actuators 21and 22 being illustrated by way of example. The pistons 23 of theactuators operate the brake shoes or brake pads, a cam arrangement 24for this purpose being shown by way of example. That part of the brakesystem of the trailer is connected to the part of the system on thetractor by means of a twin, quick-release self-sealing breakawaycoupling 25 (FIG. 1). An accumulator line 26 (FIGS. 1 and 2) leads fromthe trailer coupling head 27 to the accumulator 170 and a brake line 28leads from the trailer coupling head 29 to the hydraulic actuators 21and 22.

The hydraulic accumulator 170 is a dual purpose accumulator in that itnot only provides a source of hydraulic fluid under pressure foroperating the trailer breaks hydraulically but also serves to apply thetrailer brakes mechanically in the event of a loss of pressure from theaccumulator. The accumulator 170 comprises a fixed body 171 in one endof which a piston 172 is slidable. The rod 32 of the piston 172 isconnected by a connecting rod 33 to a yoke 34, the ends of which areconnected by brake wires 35 to the individual pistons 23 of thehydraulic actuators 21 and 22. A spring 36 acts between the body 171 anda sleeve 37 which has at one end a flange abutted by a collar 173 on thepiston 172. Thus the spring 36 urges the piston 172 to the left in adirection to apply the trailer brakes mechanically and is sufficientlypre-stressed to effect full application of the trailer brakesmechanically. A handbrake lever 38 is coupled to the connecting rod 33for applying the trailer brakes independently of the spring 36, theconnecting rod 33 having a lost motion connection 39 with the piston rod32 for this purpose. The accumulator body 171 contains a cylindricalcavity 174 which is divided into chambers 175 and 176 by a free piston177. The chamber 175 serves to store the hydraulic fluid under pressureand is connected via a bore 178 in the piston 172 to the line 26. Thechamber 176 is charged through an inlet (.not shown) with a gas under asuitable pressure to provide a gas cushion. The gas cushion can bereplaced by a compression spring acting between the piston 177 and theleft hand end of the body 171.

The accumulator chamber 175 is charged with hydraulic fluid through theunloader valve 18 and a line 44 (FIG. 1) leading from a pressure outletport 45 of the unloader valve 18 to the tractor coupling head 46 coupledto the trailer coupling head 27. The unloader valve has a discharge port47 connected by line 48 to the reservoir 15. The unloader valve containsa conical closure member 49 biassed by a spring 50 towards a valve seat51. The closure member 49 is attached to a piston 52 of slightly largerdiameter than the valve seat 51. The valve inlet chamber 53 at the lefthand side of the piston 52 is connected by a passage 54 to the inletport 19. The inlet port 19 is connected through a nonreturn valve 55 tothe pressure outlet port 45. A pressure chamber 56 at the right handside of the piston 52 is also connected to the pressure port 45. Thevalve chamber 57, serving as the outlet chamber and accommodating thespring 50, is connected to the discharge port 47.

The unloader valve 18 enables the pump 10, which may for example be agear pump and which normally operates under constant delivery quantitybut variable delivery pressure, to charge the accumulator chamber 175 toa pre-determined pressure.

When the pump 10 is first started up and assuming that none of the basictractor hydraulic services is operating, i.e., all the valves are intheir open-center or open-neutral position, the fluid delivered by thepump flows through the lines 13 and 14 to the inlet port 19 of the valve18. Initially the spring 50 holds the closure member 49 against its seat51 so that the hydraulic fluid flows past the non-return valve to thepressure outlet port 45 and through the accumulator lines 44 and 26 tothe accumulator chamber 175. The accumulator is shown in FIG. 2 in itscharge condition but initially the piston 172 is in its leftwardposition 177 is in its extreme rightward position in which it abuts astop 179 in the chamber 175. When the force of the pressure acting onthe piston 172 becomes as large as the initial force in the spring 36the piston 172 is moved to the right thereby simultaneously relievingthe trailer brakes of the mechanically applied force. When the pressureof the hydraulic fluid exceeds the pressure to which the gas cushion inthe chamber 176 is initially charged the piston 177 moves to the leftand balances the pressures in the chambers 175 and 176.

While the non-return valve 55 is open and the accumulator 170 is beingcharged with hydraulic fluid, the forces of the spring 50 and thepressure in the valve chamber 53 acting over the difference between theareas of the piston 52 and the valve seat 51 hold the closure member 49closed against its seat 51 against the force of the accumulator pressureacting in the chamber 56 upon the piston 52. When theaccumulatorpressure reaches a predetermined value determined by thespring 50 and the pressure in the chamber 53, the closure member 49 ismoved to the left thereby opening the valve to connect the inlet port 19to the discharge port 47 which is at reservoir pressure. Once the valvehas been cracked open the opening movement is rapid since the pressurein the valve inlet chamber 53 falls but the non-return valve 55 holdsthe pressure in the biasing chamber 56 at that of the accumulator. Inthe fully open position, the pressure in the chamber 56 acting on thepiston 52 has completely overcome the force of the spring 50. The valveclosure member 49 can only close again when the pressure in theaccumulator chamber 175 has fallen to a lower pressure at which it canno longer act with sufficient force on the piston 52 to overcome theforce of the spring 50. While the closure member 49 is held in itsillustrated fully open position by the pressure in the accumulatorchamber 175 the line 14 leading from the tractor hydraulic services isfreely connected to the reservoir 15 via the unloader valve 18 and thedischarge line 48. so that the tractor services can operate in thenormal manner. The non-return valve 55 also prevents the accumulatorpressure from acting on the tractor hydraulic services.

A metering valve 160 (FIG. 1) is provided on the tractor for the purposeof hydraulically actuating the trailer brakes. The metering valve 160has an inlet port 61 connected by a branch line 62 to the accumulatorline 44, a brake port 63 connected by a line 64 to a tractor couplinghead 65 coupled to the coupling head 29, and a discharge port 66connected by a line 67 to the reservoir 15. The metering valve also hasa control port 68 connected by a line 69 to the master brake cylinder(not shown) of the tractor.

The metering valve 160 comprises a body 70 containing a stepped bore. Aportion 71 of this bore receives a piston 72 whose left hand face isexposed to a control chamber 73 connected to the control port 68. Thepiston 72 abuts a second piston 74 which is slidable in a bore portion75 of slightly larger diameter than the portion 71. The right hand faceof the piston 74 bounds a brake pressure chamber 76 connected to thebrake port 63. A stem 77 on the piston 74 extends through the pressurechamber 76 and is guided in a reduced diameter portion 78 of the steppedbore. The right hand end of the reduced diameter bore'portion 78provides a first valve seat 79 while the mouth of a longitudinal passage80 in the stem 77 provides a second valve seat 81 at the free end of thestem 77. A conical valve closure member 82 cooperates with both thevalve seats 79 and 81. The closure member 82 has a stem 83 which passesthrough a larger diameter portion 84 of the stepped bore and issealingly guided in a reduced diameter portion 85 of this bore. The stem83 of the closure member 82 contains a longitudinal passage 86 whichleads from a chamber 87 at the right hand end of the stepped bore tooblique passages in the closure member 82. These oblique passages havemouths so disposed in the conical face of the closure member 82 thatthey communicate with the pressure chamber 76 whatever the positions ofthe piston 74 and the closure member 82, i.e., they open at a diameterbetween the diameters of the valve seats 79 and 81. The bore portions 78and 85 are of equal diameter so that the closure member 82 ishydrostatically balanced. The larger diameter bore portion 84 defines avalve chamber 88 connected to the inlet port 61. The longitudinalpassage 80 in the piston stem 77 is connected by transverse bores to anannular groove 90 in the piston 74. This annular groove 90 communicateswith the mouth of a passage 91 leading to the discharge port 66 at leastwhen the valve seat 81 is separated from the valve closure member 82 asillustrated.

When the tractor brakes are not applied, the master cylinder does notdevelop any pressure so that there is no pressure in the control chamber73. The pistons 72 and 74 are thus biased to their left hand endpositions (illustrated) by a spring 92 housed in the pressure chamber76. The closure member 82 is biassed against the seat 79 by a closurespring 93 housed in the valve chamber 88 and it will be seen that in theextreme leftward position of the piston 74 the valve seat 81 is spacedfrom the closure member 82 whereby the pressure chamber 76 is connectedthrough the passage 80 the annular groove 90 and the passage 91 to thedischarge port 66 leading through the line 67 to the reservoir 15. Thehydraulic actuators 21 and 22 are thereby free of pressure when thecontrol chamber 73 is not under pressure.

When the tractor brakes are applied, the master cylinder pressure istransmitted by the line 69 to the control chamber 73 and acts on thepiston 72. Since the pressure chamber 76 is under reservoir pressure(normally atmospheric pressure or some other very low pressure) thepistons 72 and 74 are urged to the right against the spring 92. When thevalve seat 81 engages the closure member 82 the pressure chamber 76 isisolated from the discharge port 66 and further movement of the pistons72 and 74 to the right lifts the closure member 82 from the valve seat79 thereby connecting the valve inlet chamber 88 to the pressure chamber76. The closure member 82 co-operating with the seat 79 meters the flowof hydraulic fluid from the accumulator chamber 175 (FIG. 2) via thelines 26, 44, 62, 64 and 28 to the hydraulic actuators 21 and 22. As thepressure in the pressure chamber 76 rises and approaches the pressure inthe control chamber 73 the pistons 74 and 72 are moved slightlyleftwards again to substantially shut off the flow between the closuremember 82 and the valve seat 79. Since the closure member 82 ishydrostatically balanced, the pressure in the pressure chamber 76 ismaintained by the metering valve proportional to the pressure in thecontrol chamber 73. The pressure applied to the hydraulic actuators 21and 22 is thereby directly proportional to the pressure developed by themaster cylinder. As the piston 74 is of only slightly larger diameterthan the piston 72, the pressure in the chamber 76 is maintained at avalue only slightly less than the pressure in the chamber 73. Ifdesired, the piston 74 could be made of slightly lesser diameter thanthe piston 72 or both pistons could be made of the same diameter. Thepistons 72 and 74 could be made integral with one another. When thebrake is released, the pressure in the control chamber 73 falls to zeroand the spring 92 urges the pistons 74 and 72 to their extreme leftwardpositions in which pressure chamber 76 is connected to the reservoir 15thereby releasing the pressure from the hydraulic actuators.

In the case wherein the tractor is fitted with purely mechanical brakesand has no master cylinder, 21 load cell can be fitted in one of thelinks of the tractor brakes. The load cell may be a hydraulic load cellconnected to the control chamber 73. Alternatively the load cell can beadapted to produce an electrical signal and for this purpose maycomprise an electrical strain gauge connected to an electrical circuitadapted to develop an electrical signal whose magnitude is proportionalto the applied brake force. In this case the metering valve is asolenoid valve connected to receive this signal and adapted to adjustthe pressure in the brake pressure chamber to correspond to the signal.

When so much hydraulic fluid has flowed from the accumulator chamber 175(FIG. 2) that its pressure acting in the chamber 56 (FIG. 1) has fallento a value at which the spring 50 can close the closure member 49 of theunloader valve 18, the accumulator is again charged by the pump 10 inthe previously described manner. Normally this lower pressure is stillhigher than the minimum pressure at which the piston 177 is urged awayfrom the stop 179 and the spring 36 of the accumulator is fullycompressed, as illustrated, with the abutment flange 94 on the piston172 abutting the body member 171 and the trailer brakes fully relievedof their mechanically applied force. However, should the pressure in thehydraulic accumulator fall substantially below this lower value, e.g.,due to a fracture in the accumulator line 44,26, the spring 36 urges thesleeve 37 and the piston 172 to the left thereby pulling on theconnecting rod 33 and the yoke 34 to apply the trailer brakesmechanically by the brake wires 35. Nevertheless sufficient hydraulicenergy is normally retained in the accumulator to permit several brakeactuations hydraulically, in the event of pump failure, before thebrakes are applied mechanically. The number of such hydraulic actuationsdepends upon the size of the accumulator.

A lever 161 (FIG. 1) is pivoted to the body 70 of the metering valve 160and acts on a plunger 162 which abuts the piston 72. The lever 161 canbe used for applying the trailer brakes independently of the tractorbrakes.

The dimensions of the hydraulic accumulator 170 (FIG. 2) including theforce of its spring 36 and the initial pressure of the gas cushion inthe chamber 176 are so chosen that the minimum pressure required in theaccumulator chamber to completely overcome the spring 36 isapproximately equal to the initial pressure in the chamber 176 at whichthe piston 177 rests against the stop 179. Thus during the initialcharging of the accumulator 170 through the line 26, trailer brakes aremechanically relieved of the force of the spring 36 before the piston177 commences to move to the left.

Although the mechanical actuator and the hydraulic actuators are shownas acting on the same brakes it may be convenient to provide separatebrakes or at least separate brake shoes or pads for actuation by thehydraulic actuators and by the dual purpose accumula- 101'.

A safety valve arrangement 100 is fitted in the trailer lines 26 and 28.This valve arrangement has a housing 101 containing passages 102 and 103which connect the lines 26 and 28 to the coupling heads 27 and 29respectively. The housing 101 also has a medial longitudinal bore 104 inwhich a plunger 105 is slideable. Transverse bores 108 and 109 intersectthe passages 102 and 103. The inner ends of the transverse bores I08 and109 are tapered to provide valve seats for balls (not shown) springbiassed against these seats. The tapered inner ends of the bores 108 and109 break into the longitudinal bore 104 and the balls protrude intothis bore when they are seated. The plunger 105 has a shallow annulargroove (not shown) normally registering with the balls to permit them toseat. When the plunger 105 is moved forwardly by means yet to bedescribed the said balls are unseated and place the passages 102 and 103in communication with one another through the bores 108 and 109 and anannular groove 113 in the plunger 105.

The forward free end of the plunger 105 is coupled by a chain 180 on ablock 121 attached to the tractor vehicle.

The coupling heads 27, 29, 46 and 65 are of conventional constructionand can be separated by a firm pull without damage in the event that thetrailer breaks away from the tractor. Each of the coupling heads isprovided with a self-closing valve whose movable valve closures member127, 128 is held open by abutment with the valve member 128, 127 in thecomplementary head when the heads 27 and 29 are fitted to theircorresponding heads 46 and 65 and is automatically closed by springswhen the coupling heads are pulled apart.

The safety valve arrangement 100 and the coupling heads 27,29, 46 and 65are described more fully in out co-pending Pat. application Ser. No.795,085 filed Jan. 29, 1969, now US. Pat. No. 3,526,438.

Should the trailer break away from the tractor vehicle, the couplingheads 27 and 29 are pulled away from the heads 46 and 65 respectively.When the heads are separated, the valve springs urge the valve closuremembers 127 and 128 against their seats in the coupling heads to closeoff the coupling heads. At the same time the tractor pulls on theplunger 15 and the plunger opens the ball valves to connect theaccumulator line 26 to the brake line 28. The hydraulic accumulatorthereby automatically operates the trailer brake actuators 21 and 22 inthe event of a trailer break-away. Finally the chain 180 breaks topermit complete separation of the plunger 105 from the block 121.

The lines 26 and 28 or the lines 44 and 64 will normally include aflexible hose portion. Should the line 26 or the line 44 and inparticular the flexible hose portion thereof be fractured, there will bean immediate loss of pressure from the pressure accumulator 170 whichwill result in the spring 36 applying the trailer brakes mechanicallythereby warning the driver that a fracture has occurred. Should the line28 or the line 64 fracture then the driver may not notice the fractureuntil he applies the brakes. However, once he applies the brakes andpressure is developed in the control chamber 73 of the metering valve,the fracture prevents a corresponding pressure from being developed inthe brake pressure chamber 76. Consequently, the closure member 82 isfully opened and the hydraulic fluid is immediately discharged from thehydraulic accumulator through the fracture and the consequent loss inpressure in the hydraulic accumulator permits the spring 36 to apply thetrailer brakes mechanically.

FIGS. 3 and 4 show another embodiment of the trailer brake system. InFIGS. 3 and parts like those of FIGS. 1 and 2 are denoted by likereference numerals and will not be described again. FIG. 3 differs fromFIG. 1 principally in that the metering valve 60 is not provided withmeans 161, 162 for applying the trailer brakes independently of thetractor brakes. For simplicity the closure members in coupling heads 27,29, 46 and 65 are not shown in FIG. 3.

The hydraulic accumulator 20 shown in FIG. 4 differs from theaccumulator 170 of FIG. 2 in that a cylindrical cavity 40 in the fixedbody 30 of the accumulator 20 is divided by a free piston 43 intochambers 41 and 42 of which the left hand chamber 41 is connected to theline 26 and serves to store the hydraulic fluid while the right handchamber 42 is charged with gas under a suitable pressure to provide agas cushion. Thus a piston 31, which is slidable in the right hand endof the body 30 and which acts on the piston rod 32 and is abutted by thesleeve 37, is exposed to the gas chamber 42 rather than the chamber(FIG. 2) for the hydraulic fluid. The gas cushion could be replaced by aspring acting between the piston 43 and the piston 31.

The accumulator 20 is shown in FIG. 4 in its charged condition butinitially the piston 31 is in its leftward position and the piston 43 isin its extreme leftward position. When the pressure of the hydraulicfluid exceeds the pressure to which the gas cushion in the chamber 42 isinitially charged the piston 43 moves to the right and balances thepressures in the chambers 41 and 42. As the size of the chamber 41increases the gas cushion is further compressed and when the force ofthis pressure acting on the piston 31 becomes as large as the initialforce in the spring 36 the piston 31 is moved to the right therebysimultaneously relieving the trailer brakes of the mechanically appliedforce.

Normal operation of the braking system of FIGS. 3 and 4 is substantiallyas previously described with reference to FIGS. 1 and 2.

When so much hydraulic fluid has flowed from the accumulator chamber 41that its pressure acting in the chamber 56 (FIG. 3)has fallen to a valueat which the spring 50 can close the closure member 49 of the unloadervalve 18, the accumulator 20 is again chargedby the pump 10. Normallythis lower pressure is still higher than the minimum pressure at whichthe spring 36 of the accumulator is fully compressed, as illustrated,with the abutment flange 94 on the piston 31 abutting the body member 30and the trailer brakes fully relieved of their mechanically appliedforce. However, should the pressure in the hydraulic accumulator fallbelow this lower value, e.g., due to a fracture in the accumulator line44, 26, the spring 36 urges the sleeve 37 and the piston 31 to the leftthereby applying the trailer brakes mechanically. Initially, with theaccumulator completely discharged, i.e., with the piston 43 fully to theleft, the gas pressure in the chamber 42 acting on the piston 31 isinsufficient to overcome the initial stress in the spring 36. When thechamber 41 has been filled to approximately one-third of its fullycharged volume the pressure has been sufficiently increased tocompletely overcome the spring 36 and completely relieve the trailerbrakes of the mechanically applied braking force.

As an alternative, the force of the spring 50 of the unloader valve 18(FIG. 3) and the dimensions of the hydraulic accumulator 20 (FIG. 4)including the force of its spring 36 and the initial pressure of the gascushion in the chamber 42 may be so chosen in relation to the quantityof hydraulic fluid consumed by the actuators 21 and 22 upon a singleactuation of the brake by actuation of the master cylinder that thepressure in the accumulator falls below the pressure at which the springbrakes mechanically. In this way the trailer brakes may be appliedsimultaneously and hydraulically.

We claim:

1. A vehicle braking system comprising, in combination, a dual purposehydraulic accumulator adapted to store hydraulic fluid under pressureand provided with a brake spring adapted to mechanically apply brakes ofthe vehicle, said brake spring being compressed to relieve said brakesof said mechanical actuation when hydraulic fluid is stored in saidaccumulator; a vehicle brake mechanism; hydraulicv actuator means foroperating said brake mechanism; and a metering valve for controllablyconnecting said hydraulic accumulator to said hydraulic actuator meansfor normal operation of the braking system.

2. A vehicle braking system according to claim 1 further comprisingsafety valve means for connecting said hydraulic accumulator directly tosaid hydraulic actuator means, independently of said metering valve, foremergency operation of the braking system.

3. A braking system according to claim 1 for a trailer vehicle adaptedto be hitched to a tractor vehicle in which said metering valve isfitted to the tractor vehicle and said dual purpose hydraulicaccumulator, said brake mechanism and said actuator means are fitted tothe trailer vehicle, said metering valve having a housing with an inletport, a brake port and a drain port and containing valve means adaptedto selectively and controllably connect said brake port to said drainport or said inlet port, and in which said braking system includes abrake line on said trailer vehicle and connected to said actuator means,first cooperating coupling means for releasably connecting said brakeport to said brake line, an accumulator line on said trailer vehicle andconnected to said accumulator and second cooperating coupling means forreleasably connecting said accumulator line to said metering valve inletport.

4. A braking system according to claim 3 in which said first and secondcooperating coupling means include trailer mounted coupling heads towhich said brake and accumulator lines are attached respectively,tractor-mounted coupling heads respectively cooperable therewith andclosure members at least in said trailer-mounted coupling heads andadapted to automatically seal off said trailer-mounted coupling headswhen the latter are detached from the tractor-mounted coupling heads andin which the braking system further includes a safety valve arrangementon the trailer vehicle and adapted to interconnect said accumulator andbrake lines in the event that the trailer vehicle breaks away from thetractor vehicle.

5. A braking system according to claim 3 in which said valve means ofsaid metering valve is adapted to control connections between said inletport and said brake port and between said brake port and said drainport, and in which said metering valve further comprises a brakepressure chamber in said housing connected to said brake port, a controlchamber in said housing adapted to be subjected to a control pressureand valve actuating means responsive to the difference between the fluidpressures in said brake pressure chamber and said control chamber foractuating said valve means.

6. A braking system according to claim 5 in which said valve actuatingmeans comprises piston means slidable in said housing and in which saidvalve means comprises a first valve seat disposed in said housingbetween said inlet and brake supports thereof, a second valve seat onsaid piston means, and a valve closure member cooperable with said firstand second valve seats, said second valve seat being connected to saiddrain port at least when said second seat is spaced from said closuremember, said piston means being movable by pressure in said controlchamber until said second valve seat engages said closure member,further movement of said piston means lifting said closure member fromsaid first seat to establish communication between said inlet and brakeports.

7. A braking system according to claim 1 in which said brake spring actson said brake mechanism in a direction to mechanically operate saidmechanism.

8. A vehicle braking system according to claim 1 further comprising aconstant volume hydraulic pump having a pump outlet; hydraulic servicesadapted to consume hydraulic power; at least one open-center typecontrol valve having an inlet port, a service port and a drain port,said service port being connected to said hydraulic services; and anunloader valve having an inlet port, a pressure port and a drain port,said pressure port being connected to said accumulator, said controlvalve and said unloader valve being connected in series by their saidinlet and drain ports to form a series circuit, said series circuitbeing connected to said pump outlet, said control valve being adapted tocontrol said hydraulic services and said unloader valve being adapted toload said pump to enable the pump to charge said accumulator and beingadapted to switch to a pump unloading state when the accumulator hasbeen charged to a predetermined pressure.

9. A vehicle braking system according to claim 1 further comprising ahydraulic pump having a pump outlet; an unloader valve having an inletport, a pressure port, a drain port, a valve seat between said inletport and said drain port and a valve closure member cooperating withsaid valve seat and movable away from said valve seat responsively to apredetermined pressure applied to said pressure port; means connectingsaid pump outlet to said inlet port; means connecting said pressure portto said accumulator and means interconnecting said inlet port and saidpressure port for charging said accumulator to said predeterminedpressure whereupon said valve closure member opens said valve seat toconnect said inlet port to said drain port to unload said pump.

10. A braking system for a trailer vehicle adapted to be hitched to atractor vehicle comprising a trailer brake mechanism; hydraulic actuatormeans on said trailer vehicle for operating said trailer brakemechanism; a hydraulic accumulator on said trailer vehicle for storinghydraulic fluid under pressure; a hydraulic pump on said tractorvehicle, said pump having a pump outlet; an unloader valve on saidtractor vehicle, said unloader valve having an inlet port, a pressureport, a drain port, a valve seat between said inlet port and said drainport and a valve closure member cooperating with said valve seat andmovable away from said valve seat responsively to a predeterminedpressure applied to said pressure port; means connecting said pumpoutlet to said unloader valve inlet port; means connecting said pressureport to said accumulator; means interconnecting said unloader valveinlet port and said pressure port for charging said accumulator to saidpredetermined pressure whereupon said valve closure member opens saidvalve seat to connect said unloader valve inlet port to said unloadervalve drain port to unload said pump; a metering valve on said tractorvehicle for controllably connecting said hydraulic accumulator to saidhydraulic actuator means for normal operation of the braking system,said metering valve having an inlet port, a brake port and a drain portand being adapted to selectively and controllably connect said brakeport to said metering valve drain port or said metering valve inletport; a brake line on said trailer vehicle and connected to saidactuator means; first cooperating coupling means for releasablyconnecting said brake port to said brake line; an accumulator line onsaid trailer vehicle and connected to said accumulator; and secondcooperating coupling means for releasably connecting said accumulatorline to said unloader valve pressure port and to said metering valveinlet port.

11. A braking system according to claim 10 in which said first andsecond cooperating coupling means include trailer mounted coupling headsto which said brake and accumulator lines are attached respectively,tractor-mounted coupling heads respectively cooperable therewith andclosure members at least in said trailer-mounted coupling heads andadapted to automatically seal off said trailer-mounted coupling headswhen the latter are detached from the tractor-mounted coupling heads andin which the braking system includes a safety valve arrangement on thetrailer vehicle and adapted to interconnect said accumulator and brakelines in the event that the trailer vehicle breaks away from the tractorvehicle.

12. A vehicle braking system comprising a vehicle brake mechanism;hydraulic actuator means for operating said brake mechanism; a dualpurpose hydraulic accumulator for storing hydraulic fluid underpressure; said hydraulic accumulator including a brake spring adapted tomechanically apply brakes of the vehicle said brake spring beingcompressed to relieve said brakes of said mechanical actuation whenhydraulic fluid is stored in said accumulator; a hydraulic pump having apump outlet; an unloader valve having an inlet port, a pressure port, adrain port, a valve seat between said inlet port and said drain port anda valve closure member cooperating with said valve seat and movable awayfrom said valve seat responsively to a predetermined pressure applied tosaid pressure port; means connecting said pump outlet to said inletport; means connecting said pressure port to said accumulator; meansinterconnecting said inlet port and said pressure port for charging saidaccumulator to said predetermined pressure whereupon said valve closuremember opens said valve seat to connect said inlet port to said drainport to unload said pump; an emergency valve controlling a connectionbetween said accumulator and said hydraulic actuator means; and meansfor actuating said emergency valve.

1. A vehicle braking system comprising, in combination, a dual purposehydraulic accumulator adapted to store hydraulic fluid under pressureand provided with a brake spring adapted to mechanically apply brakes ofthe vehicle, said brake spring being compressed to relieve said brakesof said mechanical actuation when hydraulic fluid is stored in saidaccumulator; a vehicle brake mechanism; hydraulic actuator means foroperating said brake mechanism; and a metering valve for controllablyconnecting said hydraulic accumulator to said hydraulic actuator meansfor normal operation of the braking system.
 2. A vehicle braking systemaccording to claim 1 further comprising safety valve means forconnecting said hydraulic accumulator directly to said hydraulicactuator means, independently of said metering valve, for emergencyoperation of the braking system.
 3. A braking system according to claim1 for a trailer vehicle adapted to be hitched to a tractor vehicLe inwhich said metering valve is fitted to the tractor vehicle and said dualpurpose hydraulic accumulator, said brake mechanism and said actuatormeans are fitted to the trailer vehicle, said metering valve having ahousing with an inlet port, a brake port and a drain port and containingvalve means adapted to selectively and controllably connect said brakeport to said drain port or said inlet port, and in which said brakingsystem includes a brake line on said trailer vehicle and connected tosaid actuator means, first cooperating coupling means for releasablyconnecting said brake port to said brake line, an accumulator line onsaid trailer vehicle and connected to said accumulator and secondcooperating coupling means for releasably connecting said accumulatorline to said metering valve inlet port.
 4. A braking system according toclaim 3 in which said first and second cooperating coupling meansinclude trailer mounted coupling heads to which said brake andaccumulator lines are attached respectively, tractor-mounted couplingheads respectively cooperable therewith and closure members at least insaid trailer-mounted coupling heads and adapted to automatically sealoff said trailer-mounted coupling heads when the latter are detachedfrom the tractor-mounted coupling heads and in which the braking systemfurther includes a safety valve arrangement on the trailer vehicle andadapted to interconnect said accumulator and brake lines in the eventthat the trailer vehicle breaks away from the tractor vehicle.
 5. Abraking system according to claim 3 in which said valve means of saidmetering valve is adapted to control connections between said inlet portand said brake port and between said brake port and said drain port, andin which said metering valve further comprises a brake pressure chamberin said housing connected to said brake port, a control chamber in saidhousing adapted to be subjected to a control pressure and valveactuating means responsive to the difference between the fluid pressuresin said brake pressure chamber and said control chamber for actuatingsaid valve means.
 6. A braking system according to claim 5 in which saidvalve actuating means comprises piston means slidable in said housingand in which said valve means comprises a first valve seat disposed insaid housing between said inlet and brake supports thereof, a secondvalve seat on said piston means, and a valve closure member cooperablewith said first and second valve seats, said second valve seat beingconnected to said drain port at least when said second seat is spacedfrom said closure member, said piston means being movable by pressure insaid control chamber until said second valve seat engages said closuremember, further movement of said piston means lifting said closuremember from said first seat to establish communication between saidinlet and brake ports.
 7. A braking system according to claim 1 in whichsaid brake spring acts on said brake mechanism in a direction tomechanically operate said mechanism.
 8. A vehicle braking systemaccording to claim 1 further comprising a constant volume hydraulic pumphaving a pump outlet; hydraulic services adapted to consume hydraulicpower; at least one open-center type control valve having an inlet port,a service port and a drain port, said service port being connected tosaid hydraulic services; and an unloader valve having an inlet port, apressure port and a drain port, said pressure port being connected tosaid accumulator, said control valve and said unloader valve beingconnected in series by their said inlet and drain ports to form a seriescircuit, said series circuit being connected to said pump outlet, saidcontrol valve being adapted to control said hydraulic services and saidunloader valve being adapted to load said pump to enable the pump tocharge said accumulator and being adapted to switch to a pump unloadingstate when the accumulator has been charged to a predetermined pressure.9. A vehicle braking system acCording to claim 1 further comprising ahydraulic pump having a pump outlet; an unloader valve having an inletport, a pressure port, a drain port, a valve seat between said inletport and said drain port and a valve closure member cooperating withsaid valve seat and movable away from said valve seat responsively to apredetermined pressure applied to said pressure port; means connectingsaid pump outlet to said inlet port; means connecting said pressure portto said accumulator and means interconnecting said inlet port and saidpressure port for charging said accumulator to said predeterminedpressure whereupon said valve closure member opens said valve seat toconnect said inlet port to said drain port to unload said pump.
 10. Abraking system for a trailer vehicle adapted to be hitched to a tractorvehicle comprising a trailer brake mechanism; hydraulic actuator meanson said trailer vehicle for operating said trailer brake mechanism; ahydraulic accumulator on said trailer vehicle for storing hydraulicfluid under pressure; a hydraulic pump on said tractor vehicle, saidpump having a pump outlet; an unloader valve on said tractor vehicle,said unloader valve having an inlet port, a pressure port, a drain port,a valve seat between said inlet port and said drain port and a valveclosure member cooperating with said valve seat and movable away fromsaid valve seat responsively to a predetermined pressure applied to saidpressure port; means connecting said pump outlet to said unloader valveinlet port; means connecting said pressure port to said accumulator;means interconnecting said unloader valve inlet port and said pressureport for charging said accumulator to said predetermined pressurewhereupon said valve closure member opens said valve seat to connectsaid unloader valve inlet port to said unloader valve drain port tounload said pump; a metering valve on said tractor vehicle forcontrollably connecting said hydraulic accumulator to said hydraulicactuator means for normal operation of the braking system, said meteringvalve having an inlet port, a brake port and a drain port and beingadapted to selectively and controllably connect said brake port to saidmetering valve drain port or said metering valve inlet port; a brakeline on said trailer vehicle and connected to said actuator means; firstcooperating coupling means for releasably connecting said brake port tosaid brake line; an accumulator line on said trailer vehicle andconnected to said accumulator; and second cooperating coupling means forreleasably connecting said accumulator line to said unloader valvepressure port and to said metering valve inlet port.
 11. A brakingsystem according to claim 10 in which said first and second cooperatingcoupling means include trailer mounted coupling heads to which saidbrake and accumulator lines are attached respectively, tractor-mountedcoupling heads respectively cooperable therewith and closure members atleast in said trailer-mounted coupling heads and adapted toautomatically seal off said trailer-mounted coupling heads when thelatter are detached from the tractor-mounted coupling heads and in whichthe braking system includes a safety valve arrangement on the trailervehicle and adapted to interconnect said accumulator and brake lines inthe event that the trailer vehicle breaks away from the tractor vehicle.12. A vehicle braking system comprising a vehicle brake mechanism;hydraulic actuator means for operating said brake mechanism; a dualpurpose hydraulic accumulator for storing hydraulic fluid underpressure; said hydraulic accumulator including a brake spring adapted tomechanically apply brakes of the vehicle said brake spring beingcompressed to relieve said brakes of said mechanical actuation whenhydraulic fluid is stored in said accumulator; a hydraulic pump having apump outlet; an unloader valve having an inlet port, a pressure port, adrain port, a valve seat between said inlet port and said drain port anda valve closure member coOperating with said valve seat and movable awayfrom said valve seat responsively to a predetermined pressure applied tosaid pressure port; means connecting said pump outlet to said inletport; means connecting said pressure port to said accumulator; meansinterconnecting said inlet port and said pressure port for charging saidaccumulator to said predetermined pressure whereupon said valve closuremember opens said valve seat to connect said inlet port to said drainport to unload said pump; an emergency valve controlling a connectionbetween said accumulator and said hydraulic actuator means; and meansfor actuating said emergency valve.